Mobile platforms such as iOS and Android are gaining
increasing popularity; millions of software apps on
them provide services to users. Apps often require 
access to potentially sensitive user data (e.g.,
contact list, GPS location, photos), and make
protecting user privacy a challenging task. 
To alleviate this problem, we present a 
technique (and its tool implementation, called \ourtool)
to enforce user privacy on mobile platforms.
\ourtool takes a mobile app with a set of security policies
as input, and uses a combination of static analysis and dynamic
enforcement to generate a secured version of the app.
The secured app dynamically and efficiently enforces the
specified policies, guaranteeing the security and privacy of its users.
\ourtool differs from existing approaches in two
key aspects: it requires no alterations to the operating
system or runtime; and 
it uses a static analysis to soundly identify safe
code regions that are guaranteed to comply with the
policies, and only embeds dynamic enforcement code into
the unsafe code regions. 
\ourtool' hybrid analyses make it more portable and
exhibit significant lower runtime overhead than existing
approaches.

We evaluated \ourtool by running the DroidBench Android app analysis benchmark and applying the tool to enforce privacy policies on 2 real Android malware. Our experimental
results show that \ourtool is able to precisely enforce the specified privacy policy with very little runtime overhead.

